Humidity sensing apparatus and method

ABSTRACT

The present invention is a humidity sensor which can be used in a radiosonde. The humidity sensor includes a heat sink attached to the radiosonde, a Peltier cooler attached to the heat sink, a carbon element with a thermistor attached therein or thereon, and a control means. The resistance of the carbon element is adjusted to a predetermined level to maintain a relative humidity of about 33, in particular. The control means monitors this resistance and adjusts the Peltier cooler accordingly. The thermistor responding to the temperature of the carbon element outputs a resistance indicative of the temperature on the sensor surface.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment of anyroyalty thereon.

BACKGROUND OF THE INVENTION

The present invention relates to atmospheric sciences, and, inparticular, relates to an apparatus and a method of determining dewpoint temperature and/or water vapor pressure, for example.

Radiosonde humidity measurements are routinely made with transducerswhich respond to the relative humidity of the air, i.e., carbon element,humicap sensor, lithium chloride, hair, goldbeaters skin. All aredesigned to provide a variation in some electrical quantity such asresistance or capacitance or impedance with changes in relativehumidity. The value of this electrical property is then sensed andtelemetered to the ground where it is used to compute the relativehumidity. All of these devices exhibit some temperature dependency, thatis the relationship of the electrical quantity used to relative humidityis not one curve, but a family of curves, one for each temperature.

The most commonly used sensor in the United States is the carbonelement. In radiosonde applications the carbon element is located in aduct which serves the dual purpose of shielding the element from rainand from insolation. Since the carbon element responds to relativehumidity (RH), it is necessary to know the temperature of the air tocalculate other measures of humidity, e.g., water vapor pressure,dew/frost point temperature, specific humidity, absolute humidity andmixing ratio. In some of these, the pressure of the air is also requiredfor calculation. In attempting to measure the relative humidity of theair, the defining temperature is the temperature of the surface of thesensor which thermodynamic considerations dictate to be identical withthe air temperature immediately in contact with the sensor. This surfacetemperature is in general distinct from the free air temperature, i.e.,the temperature of the air before it comes in contact with any part ofthe radiosonde or the sensor itself. In addition, any error indetermining the free air temperature can introduce error in the humidityterms computed from the relative humidity measurement.

The most basic and the largest source of error in the radiosonde carbonhumidity type of measurement is caused by the temperature differencebetween the surface of the carbon element and the free air temperature.When a parcel of air comes in contact with the sensor the temperature ofthe parcel is changed from its free air value to the sensor surfacevalue. This results in the relative humidity of that parcel beingchanged and it is this modified value of relative humidity which is thensensed by the carbon element. This error results in related errors inany of the other measures of humidity calculated using this value.

Besides the temperature effects there are two other characteristicswhich result in significant errors: the element has poor sensitivity atlow relative humidities (RH<25%) and its response time characteristicdegrades markedly at low temperatures. Because of the low sensitivity atlow RH, it is standard practice not to report humidities below 20% RH onsynoptic radiosondes. Because of the response time degradation, thedepiction of humidity features with vertical scales of less than 400 mwill be very limited at temperatures of -20° C. or lower. All three ofthe error producing characteristics can severely affect the measurementaccuracy at any altitude but their combined influence is most often feltat higher altitudes (>4 km).

The following patents are incorporated by reference as to theirteachings on humidity detection, etc.: U.S. Pat. No. 4,911,357; U.S.Pat. No. 4,801,211; U.S. Pat. No. 4,080,564; U.S. Pat. No. 4,793,182;and U.S. Pat. No. 4,793,181.

In recent years there has been an increasing requirement for moreaccurate humidity measurements. These requirements come from satelliteapplications that require improved humidity data both to calibrate andto validate their systems performance for new atmospheric models thatare sensitive to middle and upper troposphere moisture, and frommilitary applications with the increased emphasis on electro-opticalsystems. Not only is there an increased need for accuracy in general butmany of the new requirements are for increased accuracy at the higheraltitudes where the current measurements are most deficient.

SUMMARY OF THE INVENTION

The present invention provides a humidity sensing apparatus and aprocess to use such. The humidity sensing apparatus includes a humiditysensor which has a heat sink, a Peltier cooler on the top thereof, acarbon element sensor with a thermistor embedded or attached therein andthis is placed on top of the Peltier cooler.

The resistance of the carbon element is used as the control element in afeedback loop designed to control the relative humidity the carbonelement measures by controlling the temperature of the element. In thepreferred mode of operation the control circuit would maintain theelement temperature so the relative humidity would stay constant at avalue of about 33% RH. At this point the resistance of the carbonelement is invariant with temperature so that the control circuit can bedesigned to maintain the resistance of the carbon element at a constantvalue. The temperature of the carbon element is then measured by theembedded or attached thermistor and this temperature is used tocalculate the water vapor pressure of the air. This device is a watervapor pressure measuring device which can be directly converted todew/frost point information without knowledge of the ambienttemperature. In order to operate at a constant relative humidity otherthan 33%, an additional feedback from the embedded or attachedthermistor would be required to compensate for temperature effects inthe carbon element.

Therefore, one object of the present invention is to provide a humiditysensor that minimizes errors due to sensor surface temperature, lowsensitivity at low humidity, and long response time at low airtemperatures.

Another object of the present invention is to provide a humidity sensorthat operates at a constant value of relative humidity of about 33%relative humidity.

Another object of the present invention is to provide a humidity sensorthat maintains the carbon element resistance at a constant value toincrease the response time to humidity changes.

Another object of the present invention is to provide a humidity sensorhaving a temperature feedback for operating at any humidity.

These and many other objects and advantages of the present inventionwill be readily apparent to one skilled in the pertinent art from thefollowing detailed description of preferred embodiments of the inventionand the related drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a partial view of the air ducts and sensors in aradiosonde.

FIGS. 2A and 2B illustrate the humidity sensor of the present invention.

FIG. 3 is a graph of carbon element resistance versus relative humidity.

FIG. 4 is a graph of thermistor resistance versus temperature.

FIG. 5 is a graph of saturation vapor pressure versus temperature.

FIG. 6 is a circuit diagram of the control means.

FIG. 7 illustrates humidity response of the carbon element at varioustemperatures.

FIGS. 8A and 8B illustrate the control means wherein the operating pointis at any desired relative humidity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The carbon humidity element used in U.S. radiosondes was developed inthe 1950s. Since that time its manufacturing process has not beentightly controlled in that, for the most part, it has been procured by aperformance specification. Indeed, the element in use today exhibitsmuch less hysteresis and is made with a different type of carbon due toenvironmental considerations. Nevertheless, the general principal ofoperation is the same. The carbon type humidity element consists of ahumidity-sensitive film which is deposited by a spraying or dippingprocess on a base plate substrate. The electrical resistivity of thefilm varies with the humidity of the sensed environment thus providingan electrical transducer for the measurement of water vapor in theatmosphere.

In FIG. 1, in radiosonde applications the conventional carbon element 4is located in a duct 6 which serves the dual purpose of shielding theelement 4 from rain and from insolation. Since the carbon element 4responds to relative humidity, knowledge of the air temperature isnecessary to compute any of the other common measures of humidity, e.g.,vapor pressure, dew/frost point, absolute humidity. In radiosondeapplications the air temperature is measured using a thermistor 8 whichis located outside the radiosonde and supported by arms 10. The currentshape of the duct 6 was the result of work of Morrissey and Brousaides.This work quantified the magnitude of the temperature induced errorsusing the earlier ducts. The principal differences between the old andnew ducts are an extended curved exit, blackening of the inside wallsand a secondary air path 12 beneath the duct 6. While this duct reducedthe errors due to insolation it did not eliminate them and did not treatother temperature effects such as lag. This duct resulted in errors ofabout 10% of the measured value due to insolation effects above 500 mb.A similar temperature induced error above 500 mb of about 8% of themeasured value is due to the thermal lag of the element and usually hasthe same sense as the insolation error in the troposphere.

As noted above since the carbon element 4 responds to relative humidityit is necessary to know the air temperature to determine any absolutemeasure of humidity. More specifically the surface temperature of thecarbon film on the substrate itself is the defining temperature sinceheat transfer considerations dictate this to be the same as the airimmediately in contact with the surface. There are three temperaturesimportant to this measurement: the free air temperature, the surfacetemperature of the film, and the air temperature as measured by thethermistor 8. Any differences between these temperatures introduce errorin the humidity measurement. The magnitude of humidity error due totemperature differences between the carbon element 4 and the air isgiven in Table 1 where it is broken into components, insolation effectand thermal lag effect.

                  TABLE 1                                                         ______________________________________                                        Residual Temperature Induced                                                  Errors in Daytime Humidity Measurements                                                Insolation Error                                                                              Thermal Lag Error                                    Layer, mbar                                                                            (% of Measured Valve)                                                                         (% of Measured Value)                                ______________________________________                                        1013 - 701                                                                             3%              3%                                                   700 - 501                                                                              6%              4%                                                   500 - 351                                                                              9%              6%                                                   350 - 250                                                                              14%             9%                                                   ______________________________________                                    

The magnitude of error caused by temperature differences between thethermistor 8 and the air is less than the insolation and lag effectsand, during the day, of the opposite sense. For example, at 5 km thetemperature of the thermistor would be about 0.3° C. above the airtemperature resulting in about a 2% error in any absolute humiditycalculation.

The low sensitivity at low RH problem will be totally eliminated by thepresent invention in that the element will never experience low RHvalues but will be maintained at 33% RH at all times. In fact low RHvalues should be one of the most accurate areas. For example, if theelement can be maintained within 1% RH of 33% RH then if the RH of thefee air is 5% the error in measuring this should be less than 0.5% RH.It should be noted that the same effect that causes the high accuracy atlow RH will cause a loss of accuracy at high RH. If, as above, theelement can be kept within 1% RH at 33% RH, this would result in a 3% RHuncertainty for an ambient of 100% RH. This would still be animprovement on current measurements since the insolation, thermal lag,and humidity response effects will be greatly reduced.

The response of the element to change in humidity is not that of asimple first order system. Earlier researchers, Marchgraber andKobayashi, attributed this to there being a relatively fast "surface"effect and a slower volume or bulk effect. In addition the response getsmarkedly longer at lower temperatures. (FIG. 7)

The problem of increasingly long response times at low temperaturesshould also be eliminated for the most part. First the fact that it willbe part of an active servo loop will allow designing a faster response.In addition there are physical properties of the sensor that willcollaborate with the servo loop to improve the response. By designingaround 33% RH the system only has to maintain a constant carbon elementresistance. The fact that at 33% RH the resistance of the carbon elementremains constant at all temperatures indicates that the amount of waterin and on the sensor is invariant with temperature at 33% RH. Thisindicates that even though the RH of the free air changes there is nonet mass transfer to the sensor when it goes from one equilibrium stateto another. This will allow the servo loop to react to the surfaceeffects and return the sensor to equilibrium before any significantvolume effects occur. Data indicate that even at -20° C. more than 20%of the response to a step function change occurs in less than onesecond. Consequently if the Peltier cooler has sufficient capacity, a90% response to a negative step function at -20° C. in less than 5seconds is to be expected whereas the current sensor takes about 2minutes for a 90% response at the same temperature.

If absolute humidity is required, the value of air temperature is neededto compute it but the computation is much less sensitive to errors inthe measurement of the air temperature. Whereas a temperaturemeasurement error of 0.3° C. was found to cause a 2% error in absolutehumidity in the old system, a 0.3° C. error in air temperaturemeasurement for the present invention results in less than 0.2% error inabsolute humidity.

A new humidity sensing apparatus 36 and 38 are shown in FIGS. 2A and 2B,respectively. A humidity sensor 14 would be put in place of the carbonelement 4 as shown in FIG. 1.

The humidity sensor 14 of the present invention is composed: a carbonelement sensor 20 with a thermistor, not shown, either embedded, platedon, or otherwise attached thereto; a Peltier cooler 17 consisting of anupper and a lower plates 18A and 18B, respectively, and legs 19; and aheat sink 16. Control means 28 or 30 is connected to the Peltier cooler17 and the carbon element sensor 20. The carbon element sensor leads 24provide a resistance value to the control means 28 or 30. Thermistorleads 26 provide a resistance value to the output circuitry 34. Afeedback circuit 32, FIG. 2B, may be used to provide temperatureinformation to the control circuit 30 to allow for temperaturecompensation for some configurations to be discussed. With thiscompensation, constant relative humidity at the sensor 14 can bemaintained at relative humidities other than 33% RH.

Two configurations of the invention, humidity sensing apparatus 36 or 38are shown; apparatus 36, FIG. 2A, without a temperature feedback fromthe carbon element sensor 20 to the control means 28; and apparatus 38,FIG. 2B, with a temperature feedback 32 from the carbon element sensor20 to the control means 30. This temperature feedback uses theresistance value of the thermistor embedded, plated on or otherwiseattached to the carbon element sensor 20.

In general the control means 28 or 30 provides current to the Peltiercooler 17 in such a way as to heat up or cool down the carbon elementsensor 20. This causes the air in contact with the element sensorsurface to be heated or cooled which changes the relative humidity ofthe air. Heating decreases the relative humidity cooling increase therelative humidity. This change in relative humidity causes theresistance of the carbon element sensor 20 to change which is then fedinto the control means 28 or 30.

In the simpler embodiment, FIG. 2A, the circuitry used in the controlmeans 28 is designed to maintain a constant value of resistance at itsinput, which is the carbon element resistance. If this operating valueis designed to be the resistance value the carbon element has at 33% RH,then the circuitry will maintain the humidity of the air in contact withthe element constant when it keeps the resistance constant since theresistance is independent of temperature at 33% RH. The control means 28is shown in FIG. 6. The control means 28 has an AC bridge circuit 42,and a DC amplifier-driver 44. The AC bridge 42 is composed of anoscillator 46, a resistance bridge 48, and a demodulator 50. Theresistance of the carbon element is one arm of the bridge 48. Thefunction of the AC bridge 48 is to generate a DC voltage which isproportional to any change in the resistance of the carbon element. A DCbridge could also be used for this purpose. The function of the DCamplifier-driver 44 is to take the DC voltage from the AC bridge 48 andamplify it to supply sufficient DC current and of the proper sense tothe Peltier cooler 17 in the sensor to cause sufficient cooling orheating of the carbon element to return the resistance of the element tothe desired value.

The temperature of the carbon element sensor 20 can be ascertained fromthe resistance of the embedded thermistor, see formula 1. Thistemperature can be used to determine water vapor pressure e_(w) in (mb)by either looking it up in Smithsonian tables or by formulae 2 and 3.##EQU1## where T is the temperature of the thermistor in °K; T_(o) is atemperature for which the resistance is known in advance. R(T) andR(T_(o)) are the resistances of the thermistor at T and T_(o)respectively; and B is a material constant for the material of thethermistor. ##EQU2## where e_(w) is the ambient water vapor pressure inmb; T_(c) is the temperature of the carbon element in degreescentigrade; e_(s) is the saturation vapor pressure at T_(c) ; and RH isthe relative humidity expressed as a decimal. For this case RH would be0.33 (33% RH).

Dew point temperature (T_(dp)) can then be ascertained using the valueof e_(w) and looking it up in the Smithsonian tables or by formula 4:##EQU3##

Using apparatus 36 and designing it to operate at 33% RH has otheradvantages. At 33% RH, the hysteresis is small and the sensitivity, %change in resistance per % change in RH, is still reasonably large. Alsoof importance is that the amount of cooling needed for low humidities isless than if the RH were maintained at a higher value than 33% RH.

The apparatus 36 can maintain a constant resistance at a value otherthan the value at 33% RH. This would result in the relative humidity atthe sensor varying which is less desirable than the first embodiment. Itrequires an additional step in the data reduction. Once the temperatureof the carbon element sensor 20 has been calculated from the resistanceof the embedded thermistor, the relative humidity at the surface wouldhave to be determined from the calibration of the sensor, see FIG. 7, orfrom a formula representing these calibrations. There are formulascurrently in use for accomplishing this but it would depend on themanufacturer of the carbon element sensor 20. One such manufacturer isVIZ Manufacturing Co. and the formulas are shown in Tech. Publications#80415A and 80416A. Once this is accomplished the water vapor pressurecan be obtained by formula 1 where e_(s) would be ascertained as beforeusing the T_(c) and the RH would be the RH of the sensor determined asabove. The dew point temperature could be evaluated using formula 4.

A third embodiment would be using the configuration in FIG. 2B whichuses temperature compensation. This would allow the operation atconstant relative humidity at points other than 33% RH. The controlmeans would no longer be designed to maintain a constant resistance butwould maintain the resistance according to the temperature of theelement and the relative humidity that was being maintained.

FIG. 8A is a block diagram of digital means for controlling the Peltiercooler 17. Digital ohmeters 52 and 54 would measure the resistance ofthe thermistor and the carbon element which would then be input into acomputer 56; a digital-to-analog converter 58 and an amplifier-driver 60process the output of computer 56 to adjust the current to the Peltiercooler 17. An output current would drive the Peltier cooler 17. FIG. 8Billustrates by flow diagram the general processing of information in thecomputer 56. Although digital processing techniques are shown, analogdevices are possible.

Clearly, many modifications and variations of the present invention arepossible in light of the above teachings and it is therefore understood,that within the inventive scope of the inventive concept, the inventionmay be practiced otherwise than specifically claimed.

What is claimed is:
 1. A humidity sensing apparatus, said humiditysensing apparatus comprising:a heat sink; a Peltier cooler, said Peltiercooler having electrical leads for control current; a relative humiditymeasuring sensor, said relative humidity measuring sensor having achanging resistance as a function of humidity, said relative humiditymeasuring sensor mounted on top of said Peltier cooler opposite to theelectrical leads from said Peltier cooler, said relative humiditymeasuring sensor having electrical leads for outputting resistancevalues; a thermistor, said thermistor mounted in intimate contact tosaid relative humidity measuring sensor, said thermistor having achanging resistance as a function of temperature, said thermistor havingoutput electrical leads for indicating resistance values therein;control means, said control means for maintaining the relative humidityon the relative humidity measuring sensor at a predetermined value, saidcontrols means connected to said relative humidity measuring sensor,said thermistor and said Peltier cooler; and data collecting means, saiddata collecting means having input the leads from said thermistor todetermine a resistance value thereof.
 2. A humidity sensing apparatus asdefined in claim 1 wherein said relative humidity measuring sensor is acarbon element sensor.
 3. A humidity sensing apparatus, said humiditysensing apparatus comprising:a heat sink; a Peltier cooler, said Peltiercooler having electrical leads for control current; a relative humiditymeasuring sensor, said relative humidity measuring sensor having achanging resistance as a function of humidity, said relative humiditymeasuring sensor mounted on top of said Peltier cooler opposite to theelectrical leads from said Peltier cooler, said relative humiditymeasuring sensor having electrical leads for outputting resistancevalues, said relative humidity measuring sensor being a carbon elementsensor; a thermistor, said thermistor mounted in intimate contact tosaid relative humidity measuring sensor, said thermistor having achanging resistance as a function of temperature, said thermistor havingoutput electrical leads for indicating resistance values therein;control means, said control means for maintaining the resistance of saidrelative humidity measuring sensor at a predetermined value, saidcontrol means connected to said Peltier cooler and said relativehumidity measuring sensor; and data collecting means, said datacollecting means having input the leads from said thermistor todetermine a resistance value thereof whereby said relative humidity isselected in a range about 33% RH such that the resistance of said carbonelement sensor is about constant irrespective of the temperature.
 4. Aprocess of measuring air characteristics, said process comprising thesteps of:maintaining a relative humidity on a relative humiditymeasuring sensor by adjusting a temperature of a Peltier cooler indirect contact with the relative humidity measuring sensor, saidrelative humidity measuring sensor being a carbon element sensor whereinthe relative humidity is maintained in a range of about 33%; reading aresistance output of a thermistor in close contact with the relativehumidity measuring sensor; converting the resistance output to atemperature; and converting the temperature to a water vapor pressure.5. A process of measuring air characteristics, said process comprisingthe steps of:maintaining a relative humidity on a relative humiditymeasuring sensor by adjusting a temperature of a Peltier cooler indirect contact with the relative humidity measuring sensor, saidrelative humidity measuring sensor being a carbon element sensor whereinthe relative humidity is maintained in a range of about 33%; reading aresistance output of a thermistor in close contact with the relativehumidity measuring sensor; converting the resistance output to atemperature; converting the temperature to a water vapor pressure; anddetermining a dew point temperature.